The present invention relates to a method for manufacturing a magnetic tape cassette of a type which incorporates a pair of rotatable reels having a magnetic tape wound therearound, and which is formed by a combination of upper and lower half cases molded from a synthetic resin, each having a window portion. The invention also relates to a cassette manufactured by such a method.
Conventionally, audio equipment, video equipment, computers and the like have often used a magnetic tape cassette including a magnetic tape wound around a tape winding member, such as a hub or reel, to achieve recording and reproduction.
Such conventional magnetic tape cassettes are structured such that the tape winding member with the magnetic tape wound therearound is rotatably supported in a cassette case comprising upper and lower half cases, each of which is integrally formed of synthetic resin such as acrylonitrilebutadiene-styrene copolymer resin (hereinafter referred to as ABS resin) or the like.
A typical example of such a magnetic tape cassette is a tape cassette used in audio equipment. The magnetic tape cassette is normally structured in such a manner that a pair of hubs having a magnetic tape wound therearound are rotatably incorporated in and held by a case body composed of upper and lower half cases formed of ABS resin. In manufacturing the magnetic tape cassette, the upper and lower half cases are individually injection molded and are secured together by means of screws to provide a completely assembled magnetic tape cassette.
In recent years, there has been an emphasis on the aesthetic quality of the cassette case, and thus the design of the cassette case has become diversified. Accordingly, the processes used recently for manufacturing cassette cases, which are devised to obtain advantages in both design and productivity, have employed a two-color molding technique, for example.
The conventional manufacturing method using the two-color forming technique for individually forming each of the cassette halves will be described below with reference to FIGS. 4 and 5.
As shown in FIG. 5, a cassette half 40 is formed in two colors by injecting resin into an injection space 52 defined by a fixed metal mold 50, movable metal mold 51 and a slide core 54 moved by a drive device 60, and further by injecting the resin into an injection space formed upon movement of the slide core 54. In this operation, as shown in FIG. 4, there is employed a molding device which forms a plurality of the abovementioned cassette halves 40 simultaneously.
Normally, in a main body half 41 of the cassette half 40, the resin is supplied to a tooth-shaped portion 47 in an opening formed in the front portion of the cassette through a runner sprue 44 branched from a hot runner gate 43. The runner sprue 44 can be formed substantially in an H shape, as shown FIG. 4, for example. Also, as occasion demands, a hot runner gate 43 may be provided in the rear of the cassette as well, and the resin can be supplied from the rear side cassette by means of a runner sprue 45, as illustrated. Supply of the resin to a window portion 42 is performed by means of a direct hot runner gate 55, as illustrated in FIG. 5.
As shown in FIG. 5, a tunnel gate 46 is used to supply the resin to the main body half . The tunnel gate 46 has a small gate opening area, which inhibits the flow of the resin. If the gate opening area is increased, the resin flow can be enhanced. However, increasing-the gate opening area makes it more difficult to remove the resin from the gate area after the mold has been formed. As a result, when the main body half 40 is removed from the metallic mold, the load applied to the tooth-shaped portion 47 due to the gate cutting is relatively large, thereby creating the possibility of the tooth-shaped portion 47 being deformed. Accordingly, the gate opening area must be made small. As a result, if, for example, increasing the window portion 42 results in a greater number of constricted portions in the injection space which retards the flow of the resin, then the flow of the resin may be substantially worsened so as to require a runner spool 45 to be disposed in the rear of the cassette as well, as described above.
Accordingly, in spite of the fact that the cassette half formed using the above-mentioned tunnel gate 46 has its gate disposed in the tooth-shaped portion 47, the tunnel gate inhibits the flow of resin to thereby make it difficult to enhance the forming accuracy thereof. In particular, if the injection forming cycle is shortened so as to enhance the productivity thereof, then accurate formation of the cassette half is made difficult, presenting a significant problem.